For many years, it has been known that titanium dioxide can be used as a photocatalyst. A great deal of research has been done with a view toward providing photocatalytic coatings that offer self-cleaning properties. The pursuit of self-cleaning window coatings, in particular, has been an active field of exploration. These coatings typically employ a titanium dioxide layer deposited over a glass pane. The layer of titanium dioxide, which has a high refractive index, is commonly quite thick. Thick titanium dioxide layers, unfortunately, produce high levels of visible reflectance, thus creating a somewhat mirror-like appearance. This high visible reflection tends to exaggerate the appearance of dirt on a window.
Further, many known photocatalytic coating references teach that supplemental heating (or other supplemental means for delivering energy to the growing film) should be performed during and/or after film deposition so as to achieve high levels of photoactivity.
Still further, many known photocatalytic coatings have properties that are less than ideal for window applications. As noted above, the visible reflectance of known photocatalytic coatings tends to be unacceptably high. Similarly, the reflected colors of these coatings tend not to be ideal. Further, some of these coatings have particularly great surface roughness, especially those designed to have large surface areas for facilitating high photoactivity levels. Such rough coatings, unfortunately, tend to be particularly vulnerable to being abraded. They may also be particularly susceptible to taking on and stubbornly retaining dirt and other contaminants. Finally, with some photocatalytic coatings, it does not appear that the coatings will exhibit the longevity (e.g., in-field durability over time) that is desired for number-one-surface coatings.
The invention provides coatings that offer exceptional durability, exceptional optical properties, reliable production processes, and desirable cleanliness/maintenance properties. Also provided are new deposition technologies (e.g., processes and equipment) for applying thin films (regardless of coating type) to glass and other sheet-like substrates. These technologies can be used in applying a wide variety of desired coating types, and are not limited to depositing photocatalytic or hydrophilic coatings.